The invention relates to a system for modulating/demodulating digital signals for transmitting these signals using encoded modulation via a transmission channel, the said system comprising an encoding device equipped with a modulator which is situated in a transmitting station and a decoding device equipped with a demodulator which is situated in a receiving station, the encoding device comprising a channel encoder which executes an encoding for protecting the digital signals by addition of binary redundancy distributed over several character-periods of the encoded modulation, the channel encoder comprising at least one internal encoder which executes an internal convolutive encoding with efficiency R.sub.2 and which is combined with the modulation encoded by a binary allocation module.
It relates more particularly to the encoding device and the decoding device of the said system.
Such encoding and decoding devices find their applications in the transmission of digital signals through a transmission channel.
This may involve transmitting for example high-definition digital television signals through a satellite channel or digital signals by radio beams. It may likewise involve transmitting sound by mobile radio, or digital data in order to store them for example in a compact disc or a digital video recorder. In these cases it is necessary on transmission to execute at the source a prior reduction in throughput through a source encoder and to reestablish the throughput on reception through a source decoder. In the cases where it is desired to transmit digital data between two digital processing units, for example between two processors, this throughput reduction through a source encoder does not arise.
Since the most representative application is that relating to high-definition television (HDTV) for which the problems are most widespread, the present patent application is presented for this application without this constituting any limitation whatsoever.
The digitising of High-Definition Television (HDTV) generates a total unprocessed throughput of the order of 800 Mbits/sec. Transmission of this information at such a throughput cannot be economically realised in existing transmission channels. Several encoding techniques have been developed for reducing the throughput (source encoding). The performance of these encoding algorithms is measured in terms of their throughput "reduction factor" and of the quality of the picture restored after decoding. The more the redundancy of the signal is reduced, the more significant is the information transmitted. Possible transmission errors, which may be fairly easily corrected if the information transmitted is redundant, have increasingly serious consequences when the reduction factor increases.
In consequence, the transmitting of digital HDTV signals requires judicious protection. So as not to notice the effects of transmission errors on the screen, the channel error rate must be less than 10.sup.-11.
The channel used for "Direct Broadcast by Satellite" (DBS) is characterised by:
a bandwidth of 27 MHz, PA1 low power (especially for the link coming from the satellite) and the presence of a strong noise signal, regarded as additive, white and Gaussian, PA1 and nonlinear distortions. PA1 a detection module which determines for each block received a point belonging to that subset of the multidimensional constellation which is closest to the said received block, PA1 and a Viterbi decoder which operates on the sequences of the said points.
The routing of the HDTV digital signals via such a channel demands a source encoding at a significant compression rate as well as a digital modulation with high spectral efficiency.
The techniques of source encoding, based for example on an orthogonal transformation, may reduce the throughput by a factor greater than 10 while ensuring good restored picture quality. This leads to transmitting a binary throughput of the order of 80 Mbits/s. However, the transmission of such signals on a satellite channel demands a digital modulation with spectral efficiency reaching 3 bits/s/Hz.
The conventional encoding and modulating techniques prove to be limited as regards fully meeting the demands of correct transmission. However, these techniques are substantially improved by using the encoding technique proposed by G. UNGERBOECK in the article entitled: "Channel coding with multilevel/phase signals" presented in IEEE Transactions on Information Theory vol. IT-28 No. 1, January 1982, pp. 55-67.